The present invention is directed to bicycles and, more particularly, to bicycle internal hub transmissions.
Japanese Utility Model Publication 57-42792 discloses an example of a two-speed internal hub transmissions installed in the rear wheel of a bicycle. Such internal hub transmissions are well known, and they usually comprise a hub shaft structured to be mounted to the rear end of the bicycle frame, a drive member rotatably mounted around the hub shaft, a driven member also rotatably mounted around the hub shaft axially in series with the drive member, and a transmission mechanism disposed inside the drive member. A sprocket can be mounted on an outer peripheral surface of the drive member so that the drive member can be rotated by means of a chain driven by the front pedals.
The transmission mechanism usually comprises a planetary gear mechanism that communicates rotational force from the drive member to the driven member. The planetary gear mechanism typically comprises a sun gear integrally formed with the hub shaft; a planet gear carrier rotatably supported relative to the hub shaft; a planet gear rotatably supported by the planet gear carrier, wherein the planet gear engages the sun gear; and a ring gear that engages the planet gear. A clutch mechanism is provided to select desired gear ratios of the transmission mechanism.
First and second axially spaced apart bearing assemblies usually are provided between the driven member and the hub shaft so that the driven member can rotate relative to the hub shaft. Each bearing assembly comprises a bearing cone, a bearing cup mounted to the driven member, and a plurality of ball bearings disposed between the bearing cone and the bearing cup. The second bearing cone usually is formed integrally with the hub shaft. Third and fourth bearing assemblies usually are provided so that the drive member can rotate relative to the hub shaft. The third bearing assembly usually is mounted between the drive member and the driven member, and the fourth bearing assembly usually is mounted between the drive member and the hub shaft.
Since the second bearing cone and the sun gear are formed integrally with the hub shaft, they must be made sufficiently small to allow the other components to be installed during assembly. For example, when the planet gear carrier is assembled to the hub shaft, it must be passed over the second bearing cone and placed over the sun gear. As a result, the second bearing cone and the sun gear each must have an outer diameter that is smaller than the inner diameter of the planet gear carrier. While a small diameter sun gear usually poses no problem, a small diameter second bearing cone reduces the number of ball bearings that can be mounted between the second bearing cone and the second bearing cup. As a result, when the bearings are subjected to large loads, excessive pressure is applied to the bearing cone and the bearing cup, thereby shortening the service life of the bearing assembly.